Clinical isolates of Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis (TB), show diversity in transmission, pathogenicity and elicited immune responses. However, the microbial factors associated with high prevalence of particular Mtb lineages and their enhanced pathogenic potential are not well understood. Over the last two decades Mtb strains belonging to the W/Beijing lineage have become increasingly prevalent, representing 50% of isolates in East Asia and about 13% of all Mtb isolates worldwide. W/Beijing strains are also overrepresented among multidrug resistant isolates and are significantly associated with human immunodeficiency virus (HIV) infection. Thus, understanding the factors contributing to the success of this lineage is important for designing better diagnostics and intervention strategies against TB. One determinant of Mtb ability to thrive in vivo and cause disease is its response to the stringent iron (Fe) limitation imposed by the host. Fe is an essential micronutrient for Mtb proliferation that is sequestered by the host as part of nutritional immunity. Bacterial adaptation to Fe limitation is connected to virulence in two ways. First, obtaining sufficient Fe from the host is needed for bacterial proliferation. Second, Fe limitation is a signal for microbial pathogens, including Mtb, to induce production of pathogenicity factors. Our preliminary studies showed that Mtb clinical isolates are highly heterogeneous in their ability to adapt to Fe limitation in vitro. In particular, we found that a W/Beijing strain HN878, which is hypervirulent in animal models of infection, is deficient in growth in low Fe in vitro due to a natural genomic insertion of the IS6110 mobile element into a Fe storage gene bfrA. We also found that HN878 has a heightened transcriptional response to Fe limitation in vitro. In this project we propose to investigate the link between hypersensitivity to Fe deficiency and expression of virulence-related genes and pathogenicity functions in HN878. These parameters will be compared between the original HN878 strain and a HN878 strain where bfrA expression and ability to grow in low Fe have been restored. We will also examine the significance of IS6110 insertion in other members of the W/Beijing family. The results will aid in understanding the basis for Mtb heterogeneous interactions with the host and also the biological implications of IS6110 transposition ? two important aspects of Mtb biology that are largely underexplored.